The Constitution mandates that the president update Congress on the state of the union, but it says nothing about whether the media have to carry the speech.

For broadcast networks, only about half as many viewers tune in for the annual presidential speech as normally would on a given night of regular programming. Add in the fact that there are far fewer slots for commercials, and it’s simply not a profitable thing for media companies to cover.

So it’s no surprise that as broadcast networks have been bought up in recent decades by larger companies, the amount of time that they’ve allotted to covering the State of the Union address has shrunk compared with the rambling before-and-after commentaries of the 1970s.

Not so for cable.

This broadcast shrinkage has been offset by the growth of a cable news landscape that devotes hours to the event, as CNN, Fox and MSNBC battle it out in an old-fashioned ratings war for audience share. In recent years, a quickening broadband infrastructure also lets broadcast networks and cable outlets take their State of the Union analysis onto the Web once they have resumed regular programming.

“On broadcast television, where it used to be just a block that you did from 8 to 10 or from 9 to 11, now it seems that they get off the broadcast air once the Republican response is over, and they go to the website for commentary,” said Dotty Lynch, an executive-in-residence at the American University School of Communication who also works as a CBS News political consultant. “But on cable, obviously they love events like this.”

“For us, what might be different from the ’70s is the amount of broadcast time you give it — although it’s not much less, we have a number of additions that we didn’t have access to before,” said Rick Kaplan, executive producer of “CBS Evening News With Katie Couric.” “We do webcasts. We have “Washington Unplugged” in the afternoon. There’s an extraordinary amount of extra coverage given to the State of the Union.”

CNN, for example, will quite likely begin its coverage at 5 p.m. on “The Situation Room With Wolf Blitzer” and continue it through “Anderson Cooper 360” at 10 p.m. “Parker Spitzer,” at 8 p.m., will very likely be bumped by pure speech pregame, said David Bohrman, CNN’s senior vice president in charge of special events.

Bohrman previously worked on coverage of the annual speech at NBC, so he’s attuned to the stark contrast between network and cable coverage of the event.

“There was this get-off-as-fast-as-you-can mentality,” he said. “We would try to do a few minutes of analysis and try to explain, and there was tremendous pressure on all the networks. Everyone was aware of when everyone else was getting off. One of the great luxuries of cable is that you can take time to both build up and then analyze afterward. You can talk about nuance and impact and strategy and content on both sides of the speech.”

During the speech itself, networks take a big ratings hit, while cable can do better than it does on an average night. For example, during the most recent State of the Union address, 9.7 million viewers watched on Fox, 7.6 million on ABC, 7.2 million on NBC and 6.2 million on CBS, according to The Nielsen Co. These numbers are small compared with the 13 million viewers who might tune in for a successful prime-time network show such as CBS’s “Two and a Half Men” or the 20 million who tune into a blockbuster like Fox’s “American Idol.”

But on cable, the 2009 State of the Union drew 5.7 million viewers on Fox News, 3.3 million on CNN and 2.4 million on MSNBC. That’s significantly better than the roughly 3 million who routinely tune in for cable news’s top-rated prime-time show, “The O’Reilly Factor.”

Networks insist that they love the addresses, too, because coverage gives them a chance to show off their anchors and political teams to a different audience than the one watching day in, day out.

And no one has publicly tried to push back — as some networks did when they asked the White House to change the time of one of President Barack Obama’s health care addresses so they could get in an uninterrupted hour of prime-time programming.

But much as with the shrinking coverage for political conventions, the schedule doesn’t lie. Networks are just doing less of it on the broadcast airwaves, even as they add webcasts and other Internet bells and whistles to the coverage.

The biggest reason networks devote resources to the State of the Union is competition for the cache of serious and meaningful political coverage.

“There’s a tremendous responsibility on the president, and that’s a responsibility on all the news division, and it’s one of those things that our network takes very seriously,” Kaplan said.

The networks and cable outlets are still finalizing exactly who will participate in the coverage, but it’s a safe bet that each outlet’s main anchor and Sunday host will be invited to the White House for an off-the-record lunch the day of the event.

“It’s been traditional that the anchors would be invited to the White House and, in our network, Katie [Couric] and Bob Schieffer would be going,” Kaplan said.

The lunch helps guide the coverage not just of the speech but of the president for the rest of the year, according to Jon Banner, executive producer of ABC’s “World News With Diane Sawyer.”

“The anchor lunch has always been a tradition, for a couple White Houses now, and I think it’s a good opportunity to hear what’s going on in the president’s mind in a slightly less scripted way and really what they believe their goals and challenges are for the coming year.”

Recent ABC polling suggests the main concern in voters’ minds remains the economy, and there is also a lot of skepticism about whether the now-divided Congress can really work together to get things done.

“I do think that there is still, despite the tax bill, a question about can the government move as one or whether politics will rule the day,” Banner said. “That coincides with our polling. There is still a desire to get things done. The question is, will Washington be listening? It will be interesting to see what tone the president strikes, given the tone of the aftermath of the midterms. The visual of [House Speaker John] Boehner behind the president during the speech will be interesting.”

ABC News is planning to stream the speech and opposition response on its website and on Facebook, though it’s still weighing how much post-speech debate it will stream online. But ABC has a unique chance, among the networks, of being able to return to the discussion at 11:30 p.m. with “Nightline.”

In addition to expert analysis, networks and cable outlets alike usually conduct some kind of flash poll, though the results of these surveys are often controversial, because people who watch the speeches are often largely presidential supporters.

But Lynch said the instant polling provides an essential dimension of State of the Union coverage.

“It helps put some of the Beltway commentary in perspective if you have some polling from the public,” she said. “These instant polls always get attacked for being biased, but I think involving the public is a good thing.”

For special events such as the State of the Union, CNN recently began partnering with a social-media monitoring company called Crimson Hexagon, which can troll Twitter for trends of what people are talking about.

Whether through Web innovations or traditional reporting, State of the Union coverage remains important to White House staffers, because the speech can be a gift that keeps on giving — in terms of a lingering bump in opinion polls.

The best example of this phenomenon was Bill Clinton’s 1998 State of the Union address, which was delivered less than a week after the Monica Lewinsky scandal broke.

Clinton averaged a 3-percentage-point increase in approval ratings across his seven State of the Union addresses, according to the Gallup Poll, but in 1998, it jumped 10 points after he announced in the speech that the federal budget was balanced and the economy was strong.

Whether Obama also gets a bump is likely to come down to the economy, Banner said.

“It’s a big opportunity for the country to come together and figure out whether we have made progress in the last months,” he said. “I think that, ultimately, this is all about jobs.”

Concern about the economy could be good for the broadcast and cable networks carrying the speech. The highest State of the Union ratings in recent history were for President George W. Bush’s first two addresses, during times of high anxiety: in 2002, not long after the Sept. 11 attacks, and in 2003, just before the invasion of Iraq.

The 2002 speech drew nearly 52 million viewers, while the 2003 speech drew more than 62 million — far more than the 48 million who tuned in to watch Obama’s first State of the Union last year.

But the importance of the speech, for both the president and the media, extends beyond the event itself. Obama’s 2010 State of the Union was the top news story of the week, with predictions and analysis accounting for 19 percent of that week’s news hole, according to the Pew Research Center’s Project for Excellence in Journalism. That far exceeded the 11 percent generated by his September 2009 health care legislation speech or the 10 percent generated by his February 2009 speech on the economic crisis.